Josephson-like devices can be categorized by their current-voltage (I-V) characteristics as being either hysteretic or nonhysteretic. The former class includes the common three-layer superconductor-oxide-superconductor sandwich structure, whereas the latter includes weak-link microbridges, point contact devices, high current density oxide barrier junctions, and three-layer structures with semiconductor barrier layers (e.g., Si or Ge-Sn).
Josephson logic circuits are now made almost exclusively with junctions having highly hysteretic I-V characteristics. Although the hysteresis can complicate circuit design in that the junction tends to latch and thus has to be reset, it does provide two well-defined logic states. In addition, the hysteresis gives high effective current gain in the switching elements - a small control signal can result in a large output signal. In contrast, the same circuits made with nonhysteretic junctions have greatly reduced gain and ill-defined logic levels.